S. Vanden Eynde et al., Thermal behaviour of homogeneous ethylene-1-octene copolymers and linear polyethylene at high pressures, POLYMER, 41(9), 2000, pp. 3411-3423
The thermal behaviour of a linear polyethylene (LPE) and of homogeneous eth
ylene-1-octene copolymers (2.1, 5.2 and 8.0 mol% of 1-octene) was investiga
ted at high pressures by use of a high-pressure differential scanning color
imetry (DSC). The heating curves of LPE at pressures above 400 MPa reveal a
three-peak pattern (peaks denoted by I, II, III at increasing temperature)
. It reflects the melting of orthorhombic folded chain crystals (I); the me
lting of extended chain crystals, superposed on the orthorhembic-hexagonal
solid-solid transition (II) and the melting of the hexagonal phase (III). I
ntroduction of a small amount of 1-octene results in a similar multiple-pea
k pattern but with a weakened high-temperature peak. With increasing comono
mer content this three-peak pattern evolves over a two-peak pattern (I, II)
into a single-peaked endotherm (I).
The influence of the heating rate at elevated pressure as well as the influ
ence of the cooling rate at ambient pressure on the melting behaviour at hi
gh pressure are also studied. The data could be fitted well using simple eq
uations making an indicative prediction possible for the crystallisation an
d melting peak temperatures of homogeneous ethylene-1-octene copolymers as
a function of comonomer content and pressure. Besides, the possible occurre
nce of extended ethylene sequence crystals in homogeneous ethylene copolyme
rs, instead of extended chain crystals (both denoted by II) is discussed. (
C) 2000 Elsevier Science Ltd. All rights reserved.